Re: [pve-devel] [PATCH docs] pct: cgroup: update to cgroupv2 being the only supported mode

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

On 14/08/2025 13:54, Fiona Ebner wrote:
> As already mentioned in the known issues in the roadmap [0], cgroupv2
> is the only supported mode in Proxmox VE 9.
> 
> [0]: https://pve.proxmox.com/wiki/Upgrade_from_8_to_9#cgroup_V1_Removal
> 
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
> 
> Didn't want to drop mentioning the differences etc. just yet, because
> some people were still running in legacy environments.
> 
>  pct.adoc | 39 ++++++++-------------------------------
>  1 file changed, 8 insertions(+), 31 deletions(-)
> 
> diff --git a/pct.adoc b/pct.adoc
> index 529b72f..00d6329 100644
> --- a/pct.adoc
> +++ b/pct.adoc
> @@ -672,17 +672,15 @@ The main resources controlled via 'cgroups' are CPU time, memory and swap
>  limits, and access to device nodes. 'cgroups' are also used to "freeze" a
>  container before taking snapshots.
>  
> -There are 2 versions of 'cgroups' currently available,
> -https://www.kernel.org/doc/html/v5.11/admin-guide/cgroup-v1/index.html[legacy]
> -and
> -https://www.kernel.org/doc/html/v5.11/admin-guide/cgroup-v2.html['cgroupv2'].
> +The current version of 'cgroups' is
> +https://www.kernel.org/doc/html/v6.14/admin-guide/cgroup-v2.html['cgroupv2'].
> +There also is a, now unsupported,

Would word that as:

"The https://www.kernel.org/doc/html/v6.14/admin-guide/cgroup-v1/index.html[v1 version] of the cgroup subsystem was deprecated with the release of Proxmox VE 7.0 and removed entirely with Proxmox VE 9.0."

> +https://www.kernel.org/doc/html/v6.14/admin-guide/cgroup-v1/index.html[legacy]
> +version.
>  
> -Since {pve} 7.0, the default is a pure 'cgroupv2' environment. Previously a
> -"hybrid" setup was used, where resource control was mainly done in 'cgroupv1'
> -with an additional 'cgroupv2' controller which could take over some subsystems
> -via the 'cgroup_no_v1' kernel command-line parameter. (See the
> -https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html[kernel
> -parameter documentation] for details.)
> +Since {pve} 7.0, the default is a pure 'cgroupv2' environment and since {pve}
> +9.0, this is the only supported mode. Before {pve} 7.0, a "hybrid" mode was the
> +default.
>  
>  [[pct_cgroup_compat]]
>  CGroup Version Compatibility
> @@ -720,29 +718,8 @@ environment, you can either
>  * Move the container, or its services, to a Virtual Machine. Virtual Machines
>    have a much less interaction with the host, that's why one can install
>    decades old OS versions just fine there.
> -
> -* Switch back to the legacy 'cgroup' controller. Note that while it can be a
> -  valid solution, it's not a permanent one. Starting from {pve} 9.0, the legacy
> -  controller will not be supported anymore.
>  ====
>  
> -[[pct_cgroup_change_version]]
> -Changing CGroup Version
> -^^^^^^^^^^^^^^^^^^^^^^^

Maybe it would be better to keep that in for now, it's often linked too in
the forum, so would be good to just explictily state here that this is not
possible anymore with Proxmox VE 9 or later.

Alternatively we could just keep the anchor and move it earlier in the text,
where it's stated that v1 isn't supported for PVE 9 at all, that way at least
the links would still work and give the user _some_ context.

> -
> -TIP: If file system quotas are not required and all containers support 'cgroupv2',
> -it is recommended to stick to the new default.
> -
> -To switch back to the previous version the following kernel command-line
> -parameter can be used:
> -
> -----
> -systemd.unified_cgroup_hierarchy=0
> -----
> -
> -See xref:sysboot_edit_kernel_cmdline[this section] on editing the kernel boot
> -command line on where to add the parameter.
> -
>  // TODO: seccomp a bit more.
>  // TODO: pve-lxc-syscalld
>  



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